Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. User information is stored in folders of different types that include emails, calendar items, notes, contacts, and several other file formats. PST files are used for archiving emailing data offline that can be later loaded and viewed in various applications.
A file with .TXT extension represents a text document that contains plain text in the form of lines. Paragraphs in a text document are recognized by carriage returns and are used for better arrangement of file contents. A standard text document can be opened in any text editor or word processing application on different operating systems. All the text contained in such a file is in human-readable format and represented by sequence of characters.
You can also convert PST into many other file formats. Please see the complete list below.